During assembly and rework of a circuit board, the process of removing components from and reattaching components to a circuit board, a proper temperature should be maintained to permit the solder to reach and remain at its reflow temperature. For this to occur, the applied heat must be prevented from dissipating through the circuit board to adjacent components where the heat can cause potentially damaging stress to the circuit board or card and attached components.
It is common to mount complex electronic components on printed boards by inserting pins extending from components into plated through holes in the circuit board and soldering them in place. The plated through holes normally provide connections between the pins and conductive material situated at various levels of the circuit board or card.
As the number of components attached to the circuit board increases, both the number of pins and the number of times the board must be heated to solder the pins in place may increase. Therefore, during assembly and/or rework, a great amount of heat is potentially applied to a circuit board or card.
As the circuit board increases in size and thickness, the total amount of copper and the number of planes common to a single via all contribute to insufficient soldering and the inability to assemble and/or rework a component. Further, the soldering process is adversely affected when tied planes in a circuit board or circuit card, that is, planes in the circuit board or card that are electrically connected to the through hole, allow heat being applied to the plated through hole to escape into the internal planes of the circuit board. The escape of heat from the through hole reduces the through hole temperature to below the solder melting point resulting in insufficient hole fill.
The draining of heat from the through hole is especially prevalent when two or more planes are common to a single through hole. This allows heat to escape into the internal planes of the circuit board or circuit card from the plated through hole, thereby inhibiting the top surface of the carrier from reaching the solder reflow temperature. Thicker circuit boards and circuit cards that do not have common power planes may also experience assembly and/or rework problems. It may be essential that when heat is applied to the circuit board during assembly and/or rework that the temperature not be such that it could melt, burn, or char material used to form the circuit board or circuit card. However, enough heat must be applied to the solder to cause the solder to melt throughout the length of the plated through hole.
FIG. 1 illustrates a known thermal break design that may be used in the connection of plated through holes to power and ground planes in circuit boards and circuit cards to permit electrical connection between the planes and the plated through hole and minimize thermal conduction of heat during assembly and/or rework operations. The thermal break shown in FIG. 1 includes 4 spokes extending through the thermal break to electrically connect the plated through hole to power/ground planes. To be thermally effective, the spokes preferably are long, thus requiring a large area around the plate through hole or the thermal break.